This invention relates to optical metrology tools of the type described in U.S. Pat. No. 6,278,519, incorporated herein by reference. Referring to prior art FIG. 1, these types of tools include a light source for generating a probe beam 7, which is focused onto a semiconductor wafer 4. Changes between the incident probe beam 7 and the reflected beam are monitored to evaluate characteristics of the sample 4.
Tools of this type typically include a motion stage for supporting the wafer 4. Various stage motion combinations are available including full X-Y stages; R/theta stages; and ½X-½Y plus theta stages (where theta means 360 degrees of rotation). Prior art FIG. 1 exemplarily illustrates an X-stage 22, a Y-stage 24, and theta stage 26. The motion of the stages is computer controlled for moving the wafer into position with respect to the focused spot of the probe beam 7.
These tools also typically include a focusing (preferably autofocusing) system, which brings the wafer into the focal plane of the focusing optics of the measurement system 2. A number of these systems operate to translate the focusing optics in a vertical direction with respect to the sample. Alternatively, the stages themselves are provided with some form of vertical (Z-axis) movement for focusing purposes. Since the motion system needs to be designed to fit within the available height 3, conventional Z-axis stages that utilize guide rails are difficult to integrate. The length of the profile moving along the guide rails directly affects the Z-axis' stiffness against tilting movement. Where the length of the moving profile is limited by the available height, the tilting movement of the moving parts becomes hard to control. In order to reduce the tilting movement, the contact pressure between the moving profile and the guide rails needs to be increased, which results in increased friction and consequently increased actuating forces. High friction and actuating forces again reduce the movement resolution in Z-axis.
Therefore, there exists a need for an apparatus and method for highly precise vertical micro adjustment of a rotating stage with minimal friction and a maximum stiffness against tilt movement and lateral movement.
Conventional linear guiding systems define the movement direction by either a sliding or a rolling contact. This is feasible where an extensive movement range needs to be covered. In this application, the required Z-axis movement range is only twenty thousandths of an inch. Providing Z-axis movement over that small a range with sliding or rolling guides still requires a relatively bulky and heavy assembly, which increases the moment of inertia of the motion system. As a consequence, the motion system moves more slowly.
Therefore, there exists a need for a Z-axis guiding system that is low in mass as well.